What are the different Doppler measurement techniques? How do they compare?

Today there are three main types of techniques used in commercially available acoustic Doppler velocity systems. These three techniques are generally referred to as Narrowband, Broadband and Pulse-Coherent. This document gives an overview of the three techniques.

Narrowband gets its name from the bandwidth of the acoustic signal normally used in these systems. Narrowband systems operate by sending a single pulse of acoustic energy, which is relatively long (up to a few meters), and then listening to the echo these pulses generate as they bounce off particles in the water column. The frequency of the pulse is known when it leaves the transducer and its echo’s frequency is measured upon return. The difference between the transmit frequency and the return frequency is the Doppler shift, and it is proportional to the along-beam velocity of the water.

Broadband systems operate by sending two pulses of sound and listening to their echoes. A complex function is then used to calculate the phase difference between these pulses. This phase difference is proportional to the speed of the water. As they are often in the water at the same time, they are coded in order for the electronics to be able to differentiate between the two.

Similarly to Broadband, Pulse-Coherent systems operate by sending a pair of pulses, but unlike Broadband, these are relatively short and not in the water at the same time. Velocity is then measured as the phase shift between the first and second pulses of a pulse pair, which are separated by a time interval often referred to as a lag.

The different processing techniques have implications for how a system operates as they pertain to profiling range, maximum measurable velocity, short-term uncertainty (precision), power consumption, ease of operation and data output rate. These different factors will be discussed next.

Although profiling range is primarily a consequence of the instrument’s transducer frequency, transducer size (diameter) and amount of power used to transmit the acoustic pulse, it is also related the measurement technique used. For any given frequency and similar transducer size, Narrowband will generate a maximum profiling range about 20–30% greater than a Broadband system. Pulse-Coherent systems, on the other hand, have a very short profiling range because they can only transmit the second pulse in a pulse pair after the first pulse has been received. This strongly limits Pulse-Coherent systems’ ability to be effectively used in profiling systems, although this limitation is not applicable to single-point velocimeters as the distance the pulse needs to travel is already quite short (typically several centimeters only).

Similarly to profiling range, the maximum measurable velocity that a Narrowband system can detect is greater than for either a Broadband system or a Pulse-Coherent system. This is due primarily to the fact that the Narrowband technique relies on a direct Doppler shift measurement, and is therefore not limited to restrictions between transmit and receive timing that are applicable to techniques measuring phase differences between pulse pairs. Pulse-Coherent has a further limitation between range and velocity, which are inversely proportional to each other, so the faster the velocity, the shorter the profiling range that the system can measure.

For acoustic Doppler systems, precision refers to the short-term (typically 1 second) uncertainty in a measurement. Of the three techniques, Pulse-Coherent offers the best precision, followed by Broadband, followed by Narrowband. Generally speaking, for the same condition, a Pulse-Coherent system’s precision is about an order of magnitude better than a Broadband system. Narrowband systems have generally low precision and as a consequence must average data longer than either a Broadband or Pulse-Coherent system.

Power consumption is directly related to the amount of power used to transmit acoustic pulses. Therefore, it’s clear to see that Narrowband systems will generally use more power than Broadband systems as they must transmit more pulses into the water to achieve the same precision that a Broadband system would. Generally, a Narrowband system will use about 30–40% more power than an equivalent Broadband system to achieve the same precision over the same amount of time. Pulse-Coherent systems are less straightforward. Although they generally use less power than either Broadband or Narrowband systems (because their pulses are not required to travel too far), they typically will transmit much more pulses than will either Broadband or Narrowband over the same time. Pulse-Coherent systems can transmit pulses hundreds of times every second, especially when the technique is used in velocimeters rather than profilers. This then negates the gains achieved by the short profiling distance and brings its power consumption to somewhere between Narrowband and Broadband.

As far as ease of operation is concerned, Narrowband systems are by far the simplest instruments to program, operate and data-analyze. This is because the Narrowband technique is very robust and indifferent to the maximum desirable velocity and/or profiling range, provided they are within the system’s specifications. For a Broadband or Pulse-Coherent system, the maximum profiling range, velocity and precision desired all need to be carefully considered when programming the instrument and analyzing the data, as wrong configurations may lead to completely unusable data.

Lastly, when it comes to data output rate, Pulse-Coherent systems can deliver data much faster because they generally do not profile very far at all. Therefore, they do not have to wait for the return of any of the transmitted pulses in order to send another one and can ping very fast.